Cold plastic deformation of metals is a wonderful thing. It allows for processing of metal below its melting point, avoiding a host of problems with the formation of intermetallics, and oxides, and has been developed in different processes including cold spray, and friction stir joining (FSJ), including friction stir welding (FSW), and friction stir diffusion bonding (FSDB). The useful deployment of cold plastic deformation to join dissimilar metals needs to be improved, and a technique for FSJ incompatible metallic parts, especially if the metals of the two parts are prone to forming intermetallics, is needed because such intermetallics generally reduce a fatigue life of the join.
Unfortunately intermetallics are produced by joining most metals. Dissimilar material combinations shown or believed to form intermetallic compounds during FSW include: Al base alloys FSW to Fe base alloys (hereinafter two metals separated by a hyphen denote an alloy of the first FSW bonded to an alloy of the second, thus the first example is Al—Fe), Al—Mg, Al—Cu, Al—Ti, Mg—Fe, Mg—Cu, and Mg—Ti.
Al—Fe: Friction stir welding in lap joint or spot configuration has been shown to form intermetallics (Bozzi S., 2010, Intermetallic compounds in Al 6016/IF-steel friction stir spot welds; Justman R., 2013, Friction stir lap welding aluminum to steel using scribe technology). According to Bozzi, FeAl3, Fe7Al5 & FeAl2 are generated during FSW.
Al—Mg: Friction stir welding in lap joint configuration has been shown to form intermetallics (Chen Y. C., 2007, Friction stir lap joining aluminum & magnesium alloys; Liu L, 2014, A review of dissimilar welding techniques for magnesium alloys to aluminum alloys). According to Chen, brittle Mg17Al12 & gamma phase Mg2Al3 intermetallics are mostly generated during the FSW process.
Al—Cu: Friction stir welding in butt joint configuration has been shown to form intermetallics (Xue P., 2010, Enhanced mechanical properties of friction stir welded dissimilar Al—Cu joint by intermetallic compounds; Mubiayi M. P., 2013, Friction stir welding of dissimilar materials between aluminum alloys & copper—An overview). According to Xue, some intermetallics are generated during the FSW process such as Al2Cu & Al4Cu9. However, according to Mubiayi's paper, most of the structure of the weld nugget zone is largely a plastic diffusion combination of copper & aluminum.
Al—Ti: Friction stir welding in lap joint configuration has been shown to form intermetallics (Chen Y. C., 2009, Microstructural characterization & mechanical properties in friction stir welding of aluminum & titanium dissimilar alloys; Aonuma M., 2011, Dissimilar metal joining of 2024 & 7075 aluminium alloys to titanium alloys by friction stir welding). According to both papers, intermetallics (TiAl3) are generated during FSW.
Mg—Fe: Friction stir welding in lap joint configuration has been shown to form intermetallics (Chen Y. C., 2009, Friction stir lap welding of magnesium alloy & zinc-coated steel; Jana S., 2010, Friction stir lap welding of magnesium alloy to steel: A preliminary investigation). According to Chen, intermetallics are generated during the FSW process (Fe4Al13). As most Mg alloys contain appreciable amounts of aluminum as a main alloying element, these intermetallics are considered problematic for Mg—Fe.
Mg—Cu: No literature work has been found on friction stir welding of magnesium to copper due to a lack of industrial relevance. However, the phase diagram indicates that intermetallic compounds should be produced between these materials, such as CuMg2.
Mg—Ti: Friction stir welding in butt joint configuration has been shown to form intermetallics (Aonuma M., 2007, Weldability of pure titanium & AZ31 magnesium alloy by friction stir welding). According to Aonuma's paper, Al-rich titanium layer with under 2 μm in thickness was formed at joint interface. Although this paper did not use the word ‘intermetallic’ to describe the Al-rich titanium layer, it is obvious that TiAl3 intermetallic compound was generated during the FSW process. Here again the aluminum is present due to the content of the Mg alloy.
In a learned paper by Champagne III et al. entitled Joining of Dissimilar Materials by the Cold Spray Process (Proc. ITSC 2015 May 11-14, 2015, pp. 559-565), it is noted that cold spray and friction stir welding can be used for joining dissimilar metals. Specifically they report cold spraying Al onto a Mg plate, to a thickness of ˜8.5 mm (Cold Spray Processing (CSP) p. 560), and FSW joining the MG plate to an Al plate with a butt weld (FIG. 1). “Without the cold spray layer this would not be achievable without the formation of an intermetallic layer at the dissimilar metal interface.”
Unfortunately, this technique is applicable only to applications that can afford the time to deposit an 8.5 mm coating by cold spray (or at least a 5-25 mm as per FIG. 1).
Applicant has attempted to FSW join a steel part with a thinner cold spray Al coating of approximately 1 mm thickness, in butt configuration. The Al debonded during the FSW. This may be the reason why Champagne III el al. limited the coating to at least 5 mm. If a cold spray layer of at least 5 mm is required for FSW bonding, there may be no high-production-rate application for this joining, because the time required to develop a 5 mm cold spray layer is incompatible with low cost production.
A learned paper to Haghshenas et al. entitled Friction Stir Weld Assisted Diffusion Bonding of 5754 Aluminum Alloy to Coated High Strength Steels (Materials and Design (2013), doi: http://dx.doi.org/10.1016/j.matdes.2013.10.013) teaches FSDB of a high strength steel (specifically 22MnB5) that is coated with a 9 μm thick Al alloy (Al-12Si alloy). The FSDB process used brought the FSW tool “close to or nearly in contact (without penetrating) the lower steel plate”. Haghshenas et al. note:                However, the transformation of intermetallic compounds (i.e. Al5Fe2 to AlFe), is caused by change in the travel speed, and occurs in the “Al/St interface”. It should be noted that the 22MnB5 alloy could only be joined in the as-received condition, where no austenitization heat treatment was conducted. Normally this alloy is utilized as hot-stamped martensitic steel, and the Al-12Si alloy coating on the sheet material undergoes some diffusion during heat treatment prior to hotstamping. When the 22MnB5 sheets were heat treated, the appearance of the coating had changed and it would not facilitate joining when using the friction stir welding parameters studied here. Modification of this Al-12Si coating after heat treatment by light abrasion or chemical pickling with HCl solution also failed to promote adhesion during friction stir welding.An Al alloy coating, 9 μm thick, was shown by Haghshenas et al. to not shield an interface from the production of intermetallics when the pin comes close to the Al/St interface, and no value for joining can be attributed to the Al-12Si coating, however it was produced.        
One of the requirements for low cost FSW joining of dissimilar metallic parts is that the FSW tool does not enter the part having a higher resistance to plasticization. In the case of joining steel and aluminum parts, if the FSW tool enters the steel, it can improve an initial bond strength of the join, but it severely decreases a longevity and expense of the FSW tools used, as suggested by Haghshenas et al. Furthermore, the mixing of steel with aluminum leads to poor fatigue resistance for the join. There is a long-felt want for a capability to reliably join two parts of incompatible metals (especially respective ones of: steel, aluminum, and magnesium) to produce fatigue resistant joins.
Thus joins of a variety of metals that would desirably accomplished by FSW are prevented because of the formation of brittle intermetallics leads to joints that have poor longevity.